The present invention relates to a gun barrel made of a composite material and a metallic material, and specifically to a gun barrel having a metal lining and a fiber/resin composite casing which are disposed coaxially and primarily unbonded for a substantial length of the barrel so as to avoid inaccuracy and inconsistency caused by differing coefficients of thermal expansion between the metallic liner of the barrel and the composite barrel or casing.
It has long been known that forming a gun barrel out of a composite material provides advantages over traditional gun barrels made of metal. Two primary advantages are that the composite barrel is substantially lighter than the metallic barrel, and is considerably stiffer.
Typically, however, it has been found that a gun barrel which is made of both metal and a composite material is superior to those made entirely of either substance for two reasons. First, the metallic barrel liner provides a hard, machinable surface for spiral riflings in the liner bore which provide a rotational spin to the bullet during flight and greatly improves accuracy. In contrast, the composite material is not sufficiently hard, is friable, and is generally unsuitable for barrel riflings.
Second, when a bullet is fired, it is expelled from the barrel by the combustion of materials contained in the cartridge. As these materials burn, they emit gasses which force the bullet through the barrel and out an opposing end from where the cartridge is held. These gasses are extremely hot and are generally corrosive. To protect the fiber/resin composite materials from these gasses, it has become common-place to dispose a thin metallic barrel liner inside and coaxially with the composite barrel or casing material. The metallic liner of the barrel prevents the hot, corrosive gasses from contacting the composite materials, thus extending the life of the barrel.
One major problem with such metallic/composite gun barrels is that the two materials have different coefficients of thermal expansion. Due to the heat generated when firing each bullet, a barrel can quickly become warm. If rounds are repeatedly fired within a short time period, the barrel of the gun may become very hot. If the materials which form the barrel of the gun have substantially different coefficients of thermal expansion, the heat generated by repeated firing heats up the barrel which causes the metallic liner and the composite portion to expand at different rates. Those skilled in the art will appreciate that the stress developed between a metallic barrel liner bonded to a composite barrel or casing can decrease accuracy and consistency of the gun.
When a composite/metal barrel is formed, the metallic liner is generally overlaid with a composite material which has been impregnated with a binding resin, usually epoxy. The binding material solidifies the composite material to form the outer portion of the barrel or casing. The binding material will also typically bind the composite material to the metal portion. If the composite portion is formed on a mandrel, instead of directly on the metallic barrel, a bonding agent is typically used to bind the composite portion of the barrel to the metallic liner.
In such a formation, however, the bonding resin or epoxy material often prevents even contraction or expansion of the metallic liner relative to the composite portion. Often this occurs because of the differing rates of thermal expansion of the composite and metal due to the heat generated during firing. Such thermal stresses often cause the resin or bonding agent to break free of the metallic liner in a fragmented and uneven manner. When one segment of the metallic liner remains bonded to the composite portion and an opposing segment does not, the barrel will warp under the heat of firing. This decreases the accuracy of the weapon and can result in premature failure of the barrel.
Thus, there is a need for a gun barrel which incorporates the advantages of a metallic/composite gun barrel, while minimizing the problems posed by using materials which have substantially different coefficients of thermal expansion.